This invention relates to fluids and methods used in treating a subterranean formation. In particular, the invention relates to subterranean formation treatment methods and fluids using an aqueous composition of a hydrophilic polysaccharide, a crosslinker, and a breaking system. The wellbore treatment methods and fluids are particularly useful for fracturing a subterranean formation penetrated by a wellbore, resulting in very low residue in the formation after treatment, which provides improved retained conductivity.
Various types of fluids are used in operations related to the development and completion of wells that penetrate subterranean formations, and to the production of gaseous and liquid hydrocarbons from natural reservoirs into such wells. These operations include perforating subterranean formations, fracturing subterranean formations, modifying the permeability of subterranean formations, or controlling the production of sand or water from subterranean formations. The fluids employed in these oilfield operations are known as drilling fluids, completion fluids, work-over fluids, packer fluids, fracturing fluids, stimulation fluids, conformance or permeability control fluids, consolidation fluids, and the like.
A hydraulic fracturing operation is a stimulation technique routinely performed on oil and gas wells to increase fluid production from subterranean reservoirs. Specially engineered fluids are often pumped at high pressures and rates into the reservoir to be treated, causing a fracture to open. Proppants, such as ceramic beads or grains of sand, are slurried with the treating fluid (also referred to as carrier fluid) to keep the fracture propped open once the treatment is completed.
In a hydraulic fracturing operation, the fracturing fluid is a critical component of the hydraulic fracturing treatment. Its main functions are to open the fracture and to transport propping agent to the fracture. Because of their low cost, high performance and ease of handling, polymer-based fluids are the most widely used fracturing fluids. Many water-soluble polymers can be used to make a viscosified solution capable of suspending proppants, but the most widely used are guar gum and its derivatives. Guar is a long-chain, high molecular weight polymer composed of a mannose backbone and galactose side chains. The guar polymers have a high affinity for water. When the powder is added to water, the guar particles swell and hydrate. In this state, the polymer molecules are associated with many water molecules, thus unfolding and extending out into the solution, giving a viscous fluid.
Leaving a high-viscosity fluid in the fracture would reduce the permeability of the proppant pack to oil and gas, limiting the effectiveness of the fracturing treatment. Gel breakers are used to reduce the viscosity of the fluid intermingled with the proppant. For most common system, breakers reduce viscosity by cleaving the polymer chain into small molecular weight fragments. Applied to conventional polymers (i.e. guar, CMHPG, HPG, etc.), breakers leave a significant amount of insoluble polymer fragments after breaking, thus resulting in reduced retained permeability of the fracture.
There is much art dealing with the issue of improving retained permeability of the fracture. One common way to achieve a cleaner proppant pack is based on the idea of using low polymer loadings (1-3 Kg/1000 liters) to prepare a fracturing fluid. High retained fracture conductivity can also be achieved by using low molecular weight compounds as viscosifying agents in the fracturing fluids. Compounds such as de-polymerized carbohydrates or quaternary ammonium surfactants have been proven to provide a good retained conductivity, but have drawbacks such as high fluid loss into the formation, as well as high loadings of viscosifying agent to attain sufficient fluid viscosity.
Thus, the need exists for viscous fluids for oilfield treatments which provide high retained conductivity after treatment, and a fluid that can achieve this would be highly desirable. These needs are met at least in part by the following invention.